This invention relates to a oscillation isolator for mounting a precision instrument. More particularly, the invention relates to a oscillation isolator for interrupting or inhibiting the transfer of oscillation to a oscillation isolation platform from an external element such as the pedestal on which the apparatus is installed, and for making it possible to rapidly reduce and attenuate oscillation caused by operation of the device mounted on the oscillation isolation platform and to correctly maintain the oscillation isolation platform at a prescribed position and attitude.
An increase in the precision of precision devices such as electron microscopes and semiconductor aligners has been accompanied by a need to improve the performance of precision oscillation isolators on which such devices are mounted. In particular, in order to achieve proper and speedy exposure in a semiconductor aligner apparatus, an oscillation isolation platform is necessary to eliminate, to the greatest extent possible, oscillation from external sources starting with oscillation from the pedestal or floor on which the apparatus is placed. The reason for this is that oscillation which has an adverse effect upon exposure must be prevented from being produced in the stage used for exposure.
Further, in a semiconductor aligner, intermittent repetitive operations such as the step-and-repeat operation of the exposure XY stage and a scanning operation for scanning exposure induce oscillation of the oscillation isolation platform. The driving reaction force of the XY stage and movement of the load of the XY stage cause the oscillation isolation platform to vibrate. Accordingly, it is required that the oscillation isolator isolate the oscillation isolation platform from external oscillation starting with oscillation from the floor or pedestal on which the apparatus is installed, and there is a need to rapidly attenuate oscillation produced by operation of the equipment mounted on the oscillation isolation platform.
In particular, in order to perform exposure under conditions in which the exposure stage is being scanned in a scanning aligner, there is a need to improve the performance of oscillation reduction and suppression, making an oscillation isolator of improved capabilities essential.
In order to meet these demands, there has been developed and put into practical use an active oscillation isolator in which the oscillation of the oscillation isolation platform is sensed by a sensor, compensation is applied to the output signal of the sensor and the corrected signal is fed back to an actuator that applies a controlling force to the oscillation isolation platform, whereby oscillation of the oscillation isolation platform is controlled in active fashion. This active oscillation isolator makes it possible to realize excellent oscillation control performance that was difficult to achieve with the conventional oscillation isolator composed solely of passive spring elements and damper elements, etc.
Examples of active oscillation isolators that have been developed and put to use include an air-spring active oscillation isolator in which an air spring is used as an actuator, and an active oscillation isolator of the type in which an air-spring active oscillation isolator makes joint use of an electromagnetically driven actuator.
A next-generation oscillation isolator that has been proposed and developed controls the oscillation of the oscillation isolation platform using a displacement actuator such as a piezoelectric actuator, which is typified by a piezoelectric element, or a magnetostrictive actuator. An apparatus of this kind has been disclosed in, e.g., the specification of Japanese Patent Application No. 2000-136844 (an active oscillation isolator with a displacement-generating-type actuator).
This apparatus uses a piezeoelectric actuator, which is a displacement actuator exhibiting excellent controllability, thereby making it possible to easily provide a skyhook spring characteristic. A skyhook spring is a spring that acts upon the object whose oscillation is to be prevented from an absolutely stationary point in space, i.e., a spring element that does not produce direct interaction between the pedestal of the apparatus and the oscillation isolation platform. More specifically, an acceleration or velocity signal capable of being measured as a physical quantity with respect to an absolutely stationary point in space is compensated for suitably and rigidity with respect to the absolutely stationary point in space is adjusted using a control unit, whereby the skyhook function can be achieved.
In accordance with this apparatus, as described in detail also in the specification of Japanese Patent Application No. 2000-136844, the transmittance of oscillation of the apparatus pedestal in the low-frequency region to the oscillation isolation platform is made less than 0 dB by raising rigidity with respect to the absolutely stationary point, and amplitude of response with respect to external disturbance that acts directly upon the oscillation isolation platform can be reduced. The skyhook spring function is realized by using control means to adjust rigidity with respect to the absolutely stationary point in space. The displacement actuator, which is capable of adjusting amount of displacement directly and precisely, is suitable for implementing this.
This apparatus controls the amount of displacement of a displacement actuator based upon a signal corresponding to oscillational displacement of the apparatus pedestal such as the floor, thereby making it possible to readily absorb oscillational displacement of the apparatus pedestal and to interrupt the transfer of oscillation to a device mounted on the oscillation isolation platform.
Accordingly, it is possible to suppress an increase in amount of external oscillation of the apparatus pedestal even if the natural frequency of the support mechanism of the oscillation isolator is set high, i.e., even if the rigidity of the support mechanism that includes the displacement actuator is raised, in order to improve the ability to damp oscillation produced by operation of the device mounted on the oscillation isolation platform.
In other words, this apparatus has become the focus of attention as a next-generation oscillation isolator in order to enable implementation of a highly rigid oscillation isolator having excellent oscillation isolation performance.
With regard to use of a displacement actuator such as a piezoelectric actuator or magnetostrictive actuator, it is necessary to so arrange it that these actuators will not be subjected to force in a direction other than that in which displacement, which is the controlled variable, is produced. The reason for this is that a displacement actuator is easily damaged by force imposed from a direction other than a prescribed direction. In an oscillation isolator of this kind, therefore, an elastic member such as a rubber laminate exhibiting a high rigidity in the direction in which the displacement actuator acts and a comparatively low rigidity in a direction at right angles to this direction is used upon being arranged in series with the direction in which the displacement actuator acts.
However, the elastic member such as the rubber laminate is gradually deformed owing to the effect of the load. As a consequence, even though adjustments are made to position the oscillation isolation platform in a prescribed state when the oscillation isolator is installed, the position is shifted by deformation of the elastic member with the passage of time. Further, in a case where a movable mechanism such as an XY stage is mounted on the oscillation isolation platform and the platform is moved at high speed and acceleration, the platform is excited as by the driving reaction force, and displacement is produced.
In th field of precision equipment requiring an oscillation isolation platform, there are many instances where the positional relationship between the oscillation isolation platform and is peripheral equipment must be maintained in a predetermined state. This makes it necessary to provide means for correcting position and attitude of the oscillation isolation platform. Though the piezoelectric actuator and magnetostrictive actuator are actuators which produce displacement, the operating stroke thereof is not enough to correct for all displacements produced, and the state of the art is such that oscillation isolators which use displacement actuators do not posses functions for suitable position and attitude control. This is one factor that has impeded the more widespread use of oscillation isolators having a displacement actuator. Thus there is need for an oscillation isolator that can control displacement and attitude by means of a suitable arrangement while maintaining the merits of excellent oscillation preventing performance and high rigidity possessed by an oscillation isolator that uses a displacement actuator.
The present invention has been proposed to solve the foregoing problems of the prior art and an object thereof is to adapt an oscillation isolator having oscillation-proof support means which itself is devoid of a satisfactory position and attitude control function, such as an oscillation isolator that employs a displacement actuator, in such a manner that the position and attitude of a supported oscillation isolation platform can be maintained stably and with good reproducibility without sacrificing the merits of oscillation preventing performance, oscillation isolation capability and high rigidity possessed by such an oscillation isolator.
According to the present invention, the foregoing object is attained by providing an oscillation isolator having oscillation-proof support means for supporting an oscillation isolation platform, on which a device is mounted, on a pedestal while oscillation of the oscillation isolation platform is reduced, the oscillation isolator having displacement adjustment means provided between the oscillation-proof support means and the oscillation isolation platform or pedestal for adjusting relative displacement of between the oscillation-proof support means and the oscillation isolation platform or pedestal, wherein the displacement adjustment means having a rigidity higher than that of the oscillation-proof support means.
In a preferred embodiment of the oscillation isolator according to invention, the oscillation isolator further includes: detecting means for detecting relative displacement between the oscillation isolation platform and the pedestal; compensation means for producing an output signal upon compensating a detection signal output by the detection means; and drive control means for controlling the displacement adjustment means based upon the output signal of the compensation means; wherein position or attitude of the oscillation isolation platform is maintained in a predetermined state by control performed by the drive control means.
In a preferred embodiment of the oscillation isolator according to the invention, the oscillation-proof support means has a displacement actuator, wherein oscillation of the oscillation isolation platform is reduced actively by the actuator.
In a preferred embodiment of the oscillation isolator according to the invention, the oscillation-proof support means has a displacement actuator; an elastic member; an intermediate member for serially connecting the displacement actuator and the elastic member; a first oscillation sensor for sensing oscillation of the oscillation isolation platform; a second oscillation sensor for sensing oscillation of the intermediate member; oscillation compensation means for generating compensation information based upon output signals from the first and second oscillation sensors; and control means for driving and controlling the displacement actuator based upon the compensation information generated by the oscillation compensation means; oscillation of the oscillation isolation platform being reduced by control performed by the control means.
In a preferred embodiment of the oscillation isolator according to the invention, the oscillation isolator further includes a third oscillation sensor for sensing oscillation the pedestal, wherein the oscillation compensation means generates the compensation information based upon an output signal from the third oscillation sensor and output signals from the first and second oscillation sensors; and the control means controls the oscillation-proof support means so as to reduce transfer of oscillation from the pedestal to the oscillation isolation platform by driving the displacement actuator based upon the compensation information.
In a preferred embodiment of the oscillation isolator according to the invention, the displacement actuator is either a piezoelectric actuator, a magnetostrictive actuator or an ultramagnetostrictive actuator.
In a preferred embodiment of the oscillation isolator according to the invention, the displacement adjustment means includes: first and second wedge-shaped members supported so as to be movable in mutually different directions and contacting each other via oblique surfaces; a feed screw for advancing and retracting the first wedge-shaped member; and a motor for driving the feed screw; wherein the second wedge-shaped member is moved by the motor via the feed screw and first wedge-shaped member, whereby relative displacement between the oscillation-proof support means and the oscillation isolation platform or pedestal is adjusted.
In a preferred embodiment of the oscillation isolator according to the invention, the displacement adjustment means includes: a member supported so as to be movable in a predetermined direction; a feed screw for advancing and retracting the member; and a motor for driving the feed screw; wherein the member is moved by the motor via the feed screw, whereby relative displacement between the oscillation-proof support means and the oscillation isolation platform or pedestal is adjusted.
In a preferred embodiment of the oscillation isolator according to the invention, the oscillation isolator further includes: a force sensor for sensing a force that acts between the oscillation isolator and the oscillation isolation platform; and force compensation means for outputting compensation information obtained by compensating an output signal from the force sensor, wherein the displacement adjustment means is controlled, on the basis of the force compensation information, in such a manner that the force that acts between the oscillation isolator and the oscillation isolation platform will not depart from a predetermined range.
Thus, in the present invention, the displacement adjustment means adjusts the relative displacement between the oscillation-proof support means and the oscillation isolation platform or pedestal, thereby maintaining the position or attitude of the oscillation isolation platform in a predetermined state.
Accordingly, even if the oscillation-proof support means itself is devoid of a satisfactory position or attitude control capability, as in an oscillation isolator that employs a displacement actuator, the position or attitude of the oscillation isolation platform can be maintained stably in a prescribed state by the displacement adjustment means.
Further, the displacement adjustment mechanism has a rigidity sufficiently higher than that of the ordinary oscillation-proof mechanism such as rubber or springs, as a result of which no significant deformation is produced by an externally applied force. This means that the advantages of the oscillation-proof support means, namely its excellent oscillation isolating capability and high rigidity, are not sacrificed. Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.